Highly efficient α-Mn2O3@α-MnO2-500 nanocomposite for peroxymonosulfate activation: comprehensive investigation of manganese oxides

Abstract

In this paper a nanorod α-Mn₂O₃@α-MnO₂-500 nanocomposite demonstrated the highest efficiency and remarkable stability in persulfate activation compared with other manganese oxide based catalysts including α-MnO₂, β-MnO₂, γ-MnO₂, δ-MnO₂, α-Mn₂O₃, Mn₃O₄, etc. This catalyst was easily fabricated using one spot calcination treatment at 500 °C, and a minimal amount of leached Mn ions was detected during the degradation of organic contaminants. The significant performance in persulfate activation was elucidated from the unique structure and physical-chemical properties of α-Mn₂O₃@α-MnO₂-500. Evidenced by XRD and HRTEM, α-Mn₂O₃@α-MnO₂-500 consisted of a well mixed phase structure of tetragonal α-MnO₂ and cubic α-Mn₂O₃ with high crystalline quality. XPS and the inhibition effect by phosphate confirmed the existence of surface hydroxyl groups (0.926 mmol g⁻¹) on α-Mn₂O₃@α-MnO₂-500, while FTIR, Raman and ionic strength experiments further demonstrated that the formation inner-sphere interaction was the key step in PMS activation. Supported by XPS and CV, the mixed valence states in the α-Mn₂O₃@α-MnO₂-500 nanocomposite exhibited a more effective redox property, which favored electron transfer between Mn species (Mn^IV ↔ Mn^III), and generated SO₄˙⁻, ˙OH and even ¹O₂ for the degradation of various hydrocarbon contaminants. Also, the activation energy of α-Mn₂O₃@α-MnO₂-500/PMS for phenol degradation was only 24.7 kJ mol⁻¹, much lower than that of α-MnO₂ (38.7 kJ mol⁻¹) and α-Mn₂O₃ (44.9 kJ mol⁻¹). This Mn catalyst with much lower toxicity can be considered as a green approach in environmental remediation.